The fabrication of electrical devices, in particular, semiconductor integrated circuits (ICs); liquid crystal panels; organic electroluminescent panels; printed circuit boards; micro machines; DNA chips, micro plants and magnetic heads; preferably ICs with LSI (large-scale integration) or VLSI (very-large-scale integration); as well as optical devices, in particular, optical glasses such as photo-masks, lenses and prisms; inorganic electro-conductive films such as indium tin oxide (ITO); optical integrated circuits; optical switching elements; optical waveguides; optical monocrystals such as the end faces of optical fibers and scintillators; solid laser monocrystals; sapphire substrates for blue laser LEDs; semiconductor monocrystals; and glass substrates for magnetic disks; requires high precision methods which involve inter alia chemical mechanical polishing (CMP) steps using high-purity abrasive compositions. These abrasive compositions are customarily designated in the art as CMP agents or slurries.
Particular care has to be taken in the fabrication of ICs with LSI or VLSI. The semiconductor wafers used for this purpose include a semiconductor substrate such as silicon, into which regions are patterned for the deposition of different materials having electrically insulative, conductive or semiconductive properties.
In order to obtain the correct patterning, excess material used in forming the various layers on the substrates must be removed. Further, to fabricate functional and reliable ICs, it is important to have flat or planar semiconductor wafer surfaces. Thus, it is necessary to remove and/or polish certain surfaces of semiconductor wafers during the fabrication of the ICs before carrying out the next process steps.
Chemical Mechanical Polishing or Planarization (CMP) is a process in which material is removed from a substrate surface, as for example, the surface of a semiconductor wafer, and the surface is polished (planarized) by coupling a physical process such as abrasion with a chemical process such as oxidation or chelation. In its most rudimentary form, CMP involves applying a slurry, i.e., a suspension of an abrasive and an active chemistry, to a polishing pad that buffs the surface of a semiconductor wafer to achieve the removal, planarization and polishing. It is not desirable for the removal or polishing to be comprised of purely physical or purely chemical action, but rather the synergistic combination of both in order to achieve a fast uniform removal. In the fabrication of ICs, the CMP slurry should also be able to preferentially remove films that comprise complex layers of metals and other materials so that highly planar surfaces can be produced for subsequent photolithography, patterning, etching and thin-film processing.
Nowadays, copper is increasingly used for metal interconnects in ICs. In the copper damascene or dual damascene process commonly used for the metallization of circuitry in the semiconductor fabrication, the layers that must be removed and planarized include copper layers having a thickness of about 1-1.5 μm and copper seed layers having a thickness of about 0.05-0.15 μm. These copper layers are separated from the low-k and ultra low-k dielectric material by a layer of barrier material, typically about 5 to 30 nm thick, which prevents diffusion of copper into the low-k or ultra low-k dielectric material. The key to obtaining good uniformity across the wafer surface after polishing is to use a CMP slurry that has the correct removal selectivities for each material.
Although CMP processes employing CMP slurries have been and still are extensively used for fabricating electrical devices, many problems are encountered therewith.
Thus, the usual CMP process involves dispensing the CMP slurry from a stationary overlying tube dropwise onto a polishing pad of a moving, e.g., rotating, table (platen), such as one which rotates about a stationary platen axis and against which the workpiece, e.g., a wafer substrate, which is usually carried by a retaining ring, makes frictional contact while the workpiece and ring move, e.g., rotate and oscillate, relative to the platen. The workpiece is normally positioned in a medial aperture of the retaining ring. Since the position of the workpiece relative to the platen varies during wafer movement, the slurry dispensing tube is always spaced a minimum clearance distance away from the workpiece.
Consequently, different portions of the wafer or other substrate necessarily encounter dispensed slurry droplets having different chemical constitution. This depends on the continuously varying distance between the relative position of movement, e.g., rotation and oscillation, of the wafer or other substrate and in particular of its leading and trailing edges during movement and the position of the centrifugally outwardly traveling slurry droplets dispensed onto the rotating platen from the stationary tube. As a result, the amount and chemical constitution of the slurry at the local polishing site of the wafer or other substrate workpiece is inherently non-uniform, leading to non-uniformity of the CMP operation.
Also, some CMP slurry on the polishing pad is pushed off the plate by the retaining ring and wafer or other substrate workpiece arrangement, which is normally pressed under mechanical pressure (downforce) against the polishing pad. This loss of CMP slurry constitutes a wastage which increases operating costs. By its continuous sliding contact relation with the polishing pad, the wafer or other substrate workpiece necessarily impedes the flow of the CMP slurry to the central area of the surface thereof being polished. This can cause poor center-to-edge uniformity, further detracting from the uniformity of the CMP operation.
Moreover, since the CMP slurry constitutes a physical suspension of solid abrasive particles in a liquid, its components must be premixed in a storage vessel or freshly mixed immediately prior to delivery onto the polishing pad to provide the abrasive particles in the desired uniform suspension therein. If the components are premixed, the storage lifetime of the CMP slurry is a limiting factor, especially due to the potential vulnerability of the CMP slurry to agglomeration of the abrasive particles with themselves and/or with other components of the CMP slurry. On the other hand, if the components are freshly mixed immediately prior to delivery on to the polishing pad, the system must include sophisticated pumps to meter the CMP slurry in a precisely controlled flow.
But also the polishing pads customarily used in the CMP process cause problems.
Thus, their surface is roughened during the polishing by the interaction with the abrasive particles and, therefore, starts to glaze due to the incorporation of, for example, wear debris from the abrasive particles and ultra low-k materials and residues formed from metal ions and corrosion inhibitor compounds such as benzotriazole (BTA). Thus, the copper ion concentration in the CMP slurry can exceed the maximum solubility of the copper-inhibitor complexes during CMP. Therefore, the copper-inhibitor complexes can precipitate from solution and can coagulate into a surface residue. All the grit and residues can then stick to the surface of the polishing pad and accumulate to eventually filling the grooves in the polishing pads and, therefore, can deleteriously reduce the polishing efficiency. This makes it necessary to condition the polishing pads with diamond grit. However, this constitutes an additional process step which furthermore increases the operating costs.
In order to overcome the drawbacks set out above, numerous abrasive pads have been proposed in the prior art.
Thus, the international patent application WO 97/11484 and the U.S. Pat. No. 5,692,950 and U.S. Pat. No. 6,121,143 disclose abrasive articles, e.g., polishing pads, which can be erodible and which have a three-dimensional textured abrasive surface that includes a plurality of abrasive particles and a binder in a predetermined pattern. The average particle size of the abrasive particles can range from about 0.001 to 15 μm, typically between 0.01 to 10 μm. Their surface can be modified with a coupling agent which provides an association bridge between the binder and the abrasive particles.
The U.S. Pat. No. 5,733,176 discloses an abrasive polishing pad having voids and optional abrasives such as silica, ceria or zirconia incorporated therein. The voids are located beneath the polishing surface and contain an endpoint indicator substance, i.e., a fluid, for producing a detectable signal as the abrading of the abrasive polishing pad against the wafer releases the endpoint indicator, thereby indicating a worn-out polishing pad.
The U.S. Pat. No. 5,342,419 discloses an abrasive composite containing abrasive particles in a binder and clay particles dispersed therein. The composite is adhered to a backing such as paper, cloth, polymeric foam or nonwoven backing. The clay particles allow the binder, and thus the composite, to erode controllably upon abrasion of a workpiece and expose fresh abrasive particles.
The U.S. Pat. No. 5,368,619 discloses abrasive articles made from slurries of a binder precursor, i.e., a polymerizable or curable resin, abrasive particles and sufficient modifying silica particles to reduce the slurry viscosity.
The U.S. Pat. No. 5,378,251 discloses abrasive articles formed of a backing on which an adhesive coating is bonded, the coating comprising a homogeneous mixture of abrasive particles, a binder and a grinding aid consisting of a halide salt and an organic halide such as polyvinyl chloride.
The U.S. Pat. No. 5,919,082 discloses an abrasive polishing pad having a first member with an abrasive first material, e.g., a polyurethane or phenyleneoxide material containing 15-1000 nm abrasive particles such as silica, ceria, alumina, tantalum oxide or manganese dioxide, that is structurally degraded during polishing.
The U.S. Pat. No. 5,972,792 discloses an abrasive polishing pad formed of abrasive particles dispersed in a suspension medium and fixedly attached to the suspension medium.
The American patent application US 2003/0054735 A1 and the U.S. Pat. No. 6,659,846 B2 disclose an abrasive polishing pad formed of a plurality of particles of abrasive material disposed in a matrix material. The abrasive particles may be stiff inorganic materials coated with a coupling agent and the matrix material may be a polymeric material such as a polyurethane. The abrasive particles may be unevenly distributed in the matrix in order to avoid the so-called edge effect.
The American patent application US 2002/0068456 A1 discloses an abrasive polishing pad formed of abrasive particles dispersed in a polymer matrix. The polymer matrix additionally contains organic or inorganic reactive agents that can react with the metal to be polished. Examples for such organic or inorganic reactive agents are organic or inorganic nitrites and nitrates, citric acid, citrates, oxalates, tartaric acid, tartrates, amines, iodine, iodates, carbonates, diamines, ethylenediaminetetraacetic acid, ammonium compounds, chlorates, perchlorates, salicylic acid, sulfosalicylic acids, organic peroxides or inorganic peroxides such as calcium peroxide.
The American patent application US 2003/0100244 A1 and the U.S. Pat. No. 6,685,540 B2 disclose the polishing pad comprising composite particles having a solid core encapsulated by a polymeric shell material. The solid core is selected from the group consisting of ceramic materials and diamond. The composite particles are preferably spherical in shape and about 1 μm to about 2 mm in diameter. The solid core is also preferably spherical and preferably about 0.5 μm to about 0.5 mm in diameter.
The U.S. Pat. No. 7,011,574 B2 discloses an abrasive polishing pad having a polishing layer of erodible binder material containing uniformly distributed therein both abrasive particles and a water soluble ionizable electrolyte substance such as a polyelectrolyte. During the polishing, the binder material incrementally erodes, and the abrasive particles and electrolyte substance are incrementally released into direct contact with the substrate.
Although these prior art abrasive polishing pads can ameliorate to some extent the problem of locally non-uniform abrading conditions, the even flow of the CMP slurry and, thus, of the reactive agents contained therein to the center of the workpiece is still impeded, which causes poor center-to-edge uniformity, further detracting from the uniformity of the CMP operation.
There have been attempts to resolve these problems by way of polishing pads being free of abrasive particles and containing functional groups chemically or adhesively bonded to the polymeric materials constituting the polishing pads.
Thus, the German patent application DE 103 22 468 B3 discloses a polyurethane for polishing pads, which polyurethane comprises functional groups containing electron donors as ligand atoms capable of chelating or complexing copper.
The international patent application WO 2007/021414 A1 and the American patent application US 2008/0034670 A1 disclose a polishing pad containing functional groups derived from polyamines, polyelectrolytes and/or amino acids.
The U.S. Pat. No. 6,383,065 B1 discloses a polishing pad containing a catalyst having multiple oxidation states such as soluble metal compounds, in particular, iron compounds. These catalysts are capable to decompose oxidizing agents, e.g., hydrogen peroxide, contained in the CMP slurry.
These prior art polishing pads are capable to ameliorate to some extent the problems caused by the uneven flow of the agents contained in the CMP slurries. However, the problem of locally non-uniform abrading conditions still remains.